You can’t trust the seller, they just want to make the sale. I think I would find out why mapei makes that statement, and then make a decision. My guess is that the solvent in the adhesive could bleed through and discolor the stone.
It will probably be fine, they are just covering their butts. Take a piece of tile and spread adhesive on half of it. Let it dry. Look at it and see if there's any solvent or color bleed through. If not, it will be fine.
Having said that, I don't like that stuff. I use AcrylPro from Home Depot.
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